In a cipher processing system, a ciphertext file storing confidential information, etc. and a plaintext file storing nonconfidential, general information are typically managed by archiving them in different folders for easy management.
However, for example, it can be required to add a ciphertext file to a folder already containing a plaintext file. Then, the plaintext file and the ciphertext file cannot be distinguished from each other by their appearances since they have the identical extension and the like before and after ciphering if they are files created by the same application. In such a case, a plaintext file and a ciphertext file cannot be differentiated from each other, degrading convenience for a user (administrator of a file server). Additionally, as a result of the fact that a plaintext file and a ciphertext file cannot be differentiated from each other, file destruction may be caused by small carelessness such as executing a deciphering process on the plaintext file that is unnecessary for the file.
Consequently, if an identical folder contains both a ciphertext file and a plaintext file, a technique is needed for identifying both of the files and maintaining the file integrity. For example, Patent Document 1 discloses a policy to provide a label part to a header of a ciphertext file to differentiate the file from a plaintext file as a technique for permitting an identical folder to contain both a ciphertext file and a plaintext file.    Patent Document 1: JP Patent Publication (Kokai) No. 2005-165900
However, Patent Document 1 does not disclose a particular method of accessing the label part without a user or other application programs (AP) being conscious of the difference between a ciphertext file and a plaintext file. Moreover, it does not disclose a method of updating a ciphertext file while maintaining the file integrity when a user updates the file having a header provided with a label part.
In view of the above circumstances, the present invention provides a cipher processing system for allowing file access while maintaining the integrity without a user being conscious of the difference when the user accesses a file in a folder containing both a ciphertext file and a plaintext file.